This invention relates to a water drop sensor for a motor vehicle window, and particularly to such a sensor having electrically conducting electrodes separated by an insulating gap which may be spanned by a water drop. The prior art shows many examples of such sensors, which may be resistive, in which case the electrodes are placed on the outer surface of the window to be contacted directly by water drops, or capacitive, in which case the electrodes are separated from the water drops by a dielectric coating or a layer of glass so that the capacitance between the electrodes is changed by the water drops. In either case, however, an electrical parameter (resistance, capacitance) is varied by water drops on the window which overlap both electrodes. If such a sensor is placed within the wipe pattern of a vehicle window wiper system, it may be used for automatic control of wiper operation.
Sensors of this type as shown in the prior art have been shown to operate successfully at a basic level, in that they are able to distinguish between the presence or absence of moisture on the windshield and are able to activate and deactivate wiper operation or vary the wipe cycle rate or wiper speed in some manner dependent on the amount of moisture on the windshield glass in the vicinity of the sensor. The prior art shows such sensors with conducting electrodes comprising, among other materials, transparent tin oxide affixed to the windshield glass, within the wiper pattern so as to be cleared of moisture with each wipe cycle but generally located in a portion of the wipe pattern out of the main sight lines so as to minimize the visibility thereof to the vehicle occupants.
However, such prior art sensors, although operable on a basic level, have not been sufficiently sophisticated to provide optimal wiper control in a commercial product. Such prior art sensors provide a gap between the electrodes of constant width. When used for wipe cycle or speed control, such sensors exhibit decreasing sensitivity as water drop size increases, so that a larger area of windshield must be covered with precipitation to produce a given probability of sensing the precipitation. This appears to be due at least partly to the fact that the surface area of a generally circular water drop on a window increases as the square of the diameter, so that a large diameter drop is more noticeable to a vehicle occupant than a smaller diameter drop, whereas the ability to bridge the insulating gap increases only directly with the diameter.
This decrease in sensitivity with increasing drop size, for a prior art sensor with constant gap width, is illustrated in FIG. 3, wherein curves 30-32 show the percent glass coverage required to detect a predetermined number (1, 3, 8, respectively) of water drops as a function of drop diameter on the windshield. Curves 30-32 slope upward with increasing drop size, which shows that a greater portion of the window must be covered to detect the same number of drops. Thus, the upward slope of curves 30-32 means that the sensitivity of the sensor decreases with increasing drop diameter. However, one or more drops may fall on the window in the driver's line of sight before a detectable drop falls on the sensor; and each such drop affects an area of the window proportional to its area on the window. Thus, many vehicle operators would prefer a wiper control in which the sensitivity increases with increasing drop size, or at least in which the decrease in sensitivity with increasing drop size is reduced.